Optical heart rate sensors may be coupled to the user's wrist in an unpredictable way such that the strength of that coupling varies significantly from person to person and from usage to usage. When the sensor is weakly coupled, even moderate motion may cause the sensor to bounce, resulting in motion coupling in which the amount of light reflected into the sensor responds strongly to motion. When motion is periodic and overlaps the frequency range of a heart rate, motion coupling may generate peaks in the power spectrum that cannot be distinguished from spectral peaks that come from the desired photoplethysmography (PPG) signal.